The development of mobile networks gives rise to a new need in terms of synchronization to be provided for the base stations: in addition to common frequency synchronization, the base stations may need common phrase and/or time synchronization, with very restricting exactitude in the order of one microsecond.
When this type of synchronization is delivered by the network, the pieces of equipment in a packet communication network or a transport network need to have hardware functions that are dedicated to transporting a piece of synchronization information, allowing the avoidance of a transfer time variation phenomenon that is principally created by the variable waiting times for the synchronization messages in the buffer memories of the equipment.
Most of the proposed solutions use a time set protocol, such as the PTP protocol, PTP standing for “Precision Time Protocol”. Version 2 of this protocol, PTPv2, is defined by the IEEE1588-2008 standard.
In the case of the PTPv2 protocol, two types of hardware medium for the pieces of equipment in the transport network are defined:
“Boundary Clock”: a hardware medium for a clock of this type is made of PTP ports, one of which is in a “slave” state and the others are in a “master state”. A synchronization reference is obtained from synchronization messages that are received on a “slave” PTP port and is kept locally to the equipment. This synchronization reference is redistributed to other pieces of equipment: new synchronization packets are generated and sent from “master” PTP ports.
“Transparent Clock”: the packets carrying synchronization messages are routed like the other traffic packets through the equipment of the transport network, but the transit time for some of these packets carrying the synchronization messages through the equipment, called the “residence time”, is determined precisely and is conveyed by modifying the packet carrying the synchronization message at the output of the equipment. In practice, the sum of the “residence times” is indicated in a particular field of the PTPv2 synchronization message, called the “correction field”, and allows the recipient equipment to determine the variation in the transfer time from the timestamps contained in the PTPv2 synchronization message. The PTPv2 synchronization message can thus be processed as if it had not undergone a transfer time variation through the pieces of equipment of the network.
The synchronization messages are sent by a master PTP port on a piece of equipment in the network to a slave PTP port on another piece of equipment in the network. The steps below are implemented on a piece of intermediate equipment in the network, having a hardware medium for a PTP of “Transparent Clock” type:                detection of a PTPv2 synchronization message in a packet stream received on an activated PTPv2 port and timestamping of the packet at the instant of arrival;        routing of the packet carrying the PTPv2 synchronization message, that is to say reading of the header of the packet in order to determine the output port(s) to which the packet needs to be routed. In this case, it is emphasized that it is routed like the other received packets, that is to say by using the packet transfer mode supported by the equipment;        for each determined output port, modification of the “correction field” field in the payload of the packet carrying the synchronization message in order to add thereto the transit time “residence time” measured from the instants of arrival and of sending of the packet followed by sending of the modified packet to the output port.        
The synchronization messages can be transmitted in multicast mode or else in point-to-point (unicast) mode with an indication of the address of the slave PTP port in the transport network. This type of hardware medium for PTP nevertheless has a certain number of disadvantages.
The intermediate equipment needs to modify the <<correction field>> field directly in the payload of the packet carrying the PTPv2 synchronization message even though it is not the recipient of this packet. It thus intervenes beyond the layers that it needs to process in order to route the packet. This action corresponds to a layer violation at protocol level.
Moreover, the intermediate equipment needs to be capable of determining the location in this payload of the field to be modified, which in some cases, for example when there is intermediate encapsulation for routing the packet, is not immediate. It is thus necessary to indicate the type of encapsulation used to said intermediate equipment by means of configuration.
It is likewise emphasized that, for some interchanges, the paths followed by the PTPv2 synchronization messages in the downward direction from the master PTP port to the slave PTP port and in the upward direction must be identical so as not to introduce an imbalance which is restricting in an operational network.